A common difficulty associated with the use of coherent light sources such as lasers in imaging optical systems is a phenomenon known as speckle. Speckle arises when coherent light scattered from a rough surface is detected by an intensity detector that has a finite aperture, such as an observer's eye or a detector. The image on the screen appears to be quantized into little areas with sizes equal to, the detector resolution spot. The detected spot intensity vanes randomly from darkest, if contributions of the scattering points inside the spot interfere destructively, to brightest if they interfere constructively. This spot-to-spot intensity fluctuation is referred to as speckle. The resultant speckled light signal on the detector appears as spatial and temporal noise in whatever sensor is used in the imaging system.
Speckle reduction is known to involve averaging a number of independent speckle configurations, i.e. obtained from different un-correlated and non-interfering reflecting beams. Since speckle depends on essentially three light parameters: angle, polarization, and wavelength of the illuminating laser beam, independent speckle configurations can be generated through the diversification of any of these three light parameters. To solve the problem of speckle, many attempts have been made, mostly based on angle diversification, obtained by means of diffusers and/or movable optical elements, or by means of polarization diversification.
In U.S. Pat. No. 4,155,630 to Ih, there is disclosed a process and apparatus for improving image creation in a coherent light imaging system which involves directing a diffused light onto a mirror having a rocking motion whereby angle diversification is obtained. The rocking motion causes the reflected rays to sweep a two-dimensional area and focus the reflected light through a diffuser before collimating the rays for use in image creation. Applying a combination of voltages to three independent piezo-electric crystals upon which the mirror is mounted produces the rocking motion of the mirror.
U.S. Pat. No. 6,081,381 to Shalapenok, et al., describes a method and apparatus for eliminating speckle in an optical system by angle diversification obtained by the use of a diffuser and by a rotating micro-lens array having a rotational speed related to the laser parameters. The micro-lens illumination comes off of a stationary diffuser and eventually provides a large area that is uniform and speckle free illumination. U.S. Pat. No. 4,511,220 to Scully, discloses a laser target speckle eliminator for laser tight reflected from a distant target whose roughness exceeds the wavelength of the laser light. The apparatus includes a half-plate wave member, a first polarizing beam splitter member, a totally reflecting right angle prism, and a second polarizing beam splitter member, all of which are in serial optical alignment. Used in combination, the components convert a linearly (i.e., vertically) polarized laser light beam having a known coherence length, into two coincident orthogonally polarized beams that are not coherent with each other.
The resultant beams have an optical path difference exceeding the known coherence length of the laser, thereby eliminating the speckle in that system.
In U.S. Pat. No. 6,577,394 to Zavislan, there is disclosed a scanning laser confocal microscopy system for reducing speckle from scatterers that exist outside (above and below) the section which is being imaged by utilizing orthogonally polarized sheared beams. The sheared beams are focused to spots that are laterally or vertically offset. The polarized beams have opposite senses of circular polarization.